1. Field of the Invention
The present invention relates to an electrical apparatus connected with a battery charger system, particularly to a small type of electrical apparatus connected with a battery charger system such as an electric shaver, an electric toothbrush or the like.
2. Description of the Prior Art
In a small type of electrical apparatus such as an electric shaver or the like characterized by being used in a cordless state, being repeatedly used and being small and light, the battery charger system is one of the important components on which the efficiency of the apparatus depends, and it is desirable that the charger system be a fast battery charger system for charging a battery in a short time.
In a fast rechargeable battery charger system for charging a rechargeable cell such as a nickel-cadmium cell or the like which is nearly fully-charged during a short time of a few minutes, the charging current is so large that the cell reaches an overcharged state, which may make the cell inferior or damage it, when the charging current continues to be supplied to the fully-charged cell.
In order to prevent such overcharging in a conventional fast battery charger system, the prior art has proposed systems for detecting the voltage when the cell voltage is constant and damping an oscillation of a switching transistor comprising an inverter so as to convert large current charging to minimal current charging (e.g. the Japanese published unexamined patent application No. 212333/1983), as well as prior art systems for interrupting the charging or converting large current charging to minimal current charging at a predetermined point in time after the beginning of the charging which point is determined by using a timer IC (e.g. the Japanese published unexamined patent application No. 101537/1982).
However, both of these prior art systems are designed to utilize a timer IC for setting up a charging time and to control charging current by an output signal of the timer IC. Such a control timer IC or the like has a size limit which is standarized or it is more expensive than the other electrical components of the charger, so that, in either event, it is not suitable for a small type of battery charging apparatus.
In the case of a battery charger system for a small electrical apparatus, such as an electric shaver or the like, it is necessary to rapidly charge the quantity of cell capacity sufficiently to correspond with one working time, e.g. one shaving time. In such an instance, a capacitor type of timer has been utilized because it can predetermine the time required for fast charging energy of one working time and further it has a simple construction of the type suitable for a small type of electrical apparatus. However, in this case, there is a need to make the discharging resistance large in order to stabilize the operation of the capacitor used for the timer. As a result, a long interval is required for the discharge of the capacitor and, thus, it is necessary to wait for a long interval until the next fast charging can be carried out. Consequently, it takes a lot of time to re-start the timer and, thus, this method cannot serve to continually operate the timer. Furthermore, this method is also disadvantageous in that this so called fast charging can only occur about once per one day or the like. Thus, it is difficult to utilize this kind of timer for a small-sized battery-charging type of apparatus.
Another type of prior art battery-charging apparatus has generally been provided in which an error-charging prevention mechanism is employed for preventing load from the erroneous operation.
For example, what is disclosed in the Japanese published examined utility model application No. 39367/1975 shows this technique applied to an electric shaver. Namely, it discloses an arrangement in which a power plug is mechanically linked with an operation switch so as to turn off the operation switch inserted between a cell and a motor in response to the operation of projecting the power plug contained in the apparatus. However, since this kind of mechanism is complicated and is difficult to construct during manufacture, it is not preferred.
Moreover, the switching circuit normally employed for a small-sized electrical apparatus such as an electric shaver or the like conventionally has a simple construction. For example, the motor driving circuit disclosed in the Japanese published examined utility model No. 31611/1979 has a very simple construction of only a switch inserted between a power battery and a motor. Namely, most of the prior art switching circuits are occupied with constructions utilizing simple switches such as a slide switch, a push-button switch or the like.